Cancer Chemotherapy 1

considerations to chemotherapy

• long-term gain vs. risk

• probability of treatment success vs. quality of life

considerations of patient

• type/stage of cancer

• health of patient (kidney/liver function, bone marrow reserve, concurrent medical problems)

• desire to undergo difficult/dangerous treatment 

• ability to cope with side-effects

• pre-treatment screening

treatment regimen

• most given in combination (synergistic, different mechanisms of action and resistance) 

• drugs should be given as frequently and as possible close to the maximal effective dose as possible

dosage

• generally based on body surface area

• pharmacokinetics, drug interactions and impact on liver, kidneys and immune system need to be considere

DNA alkylating agents mechanism of action

• transfer alkyl group to cellular constituents 

what is the major site of DNA alkylation 

Major site in DNA: N7 and/or O6 of guanine

how many locations do they alkylate at

monofunctional (alkylate single DNA strand) or bifunctional (alkylate at two locations, cross-link)

what kinds of cells are more sensitive to DNA alkylation

proliferating cells more sensitive

major classes of DNA alyklators

Alkylsulfonates

methyl/ethylenimines

nitrogen mustards (clyclophosphamide) 

nitrosoureas

*platinum compounds (cisplatin) 

Triazenes

*technically not alkylating agents but also bind N7 and cross-link DNA

what is the most commonly used alkylating agent

cyclophosphamide

what kind of drug is a cyclophosphamide

• pro-drug, administered IV or orally, activated by cytochrome P450, lipid soluble

spectrum of activity for cyclophosphamide

• broad spectrum: used alone or in combination with other drugs to treat neuroblastomas, lymphomas, leukemias and colon, breast, ovarian, small cell lung and testicular cancers

adverse effects of cyclophosphamide

• less toxic than some alkylating agents due to cellular metabolism of aldophosphamide by aldehyde dehydrigenase

• dose-dependent Gi disturbances, bone marrow suppression, immunosuppression, hair loss, hemorrhagic cystitis (acrolein accumulation) 

• increased risk of sterility, menopause, cancer

resistance mechanisms of cyclophosphamide

• reaction with other cellular constituents

• increased metabolism (ALDH, GST)

• increased DNA repair e.g. cancer cells with high levels of O6-methylguanine-DNA methyl-transferase (MGMT) less susceptible

What is Cisplatin and what does it do

inorganic metal: covalently binds N7 and O6 of guanine also interacts with cytosine and adenine

how is cisplatin administered

IV; particularly effective for testicular, bladder and ovarian cancer.

also used to treat lymphomas, sarcomas and lung carcinomas

adverse effects of Cisplatin

• bone marrow suppression

• anemia

• GI distress (one of most emetogenic chemotherapies)

• nephrotoxicity 

• electrolyte imbalances

• neurotoxicity (hearing loss, peripheral neuropathy)

Cisplatin resistance mechanisms

• decreased access to DNA

• increased DNA repairX

explain this diagram

1. outside the cell: Cisplatin is stable

• in the bloodstream, chloride concentration is high (~100mM) 

• at high chloride levels, cisplatin stays in its neutral, stable form: Pt(NH3)2Cl2

2. entry into the cell - Cisplatin gets in two ways

a. transporter-mediated influx

• transporters like CTR1 (copper transporter 1) bring cisplatin into the cell

b. passive diffusion 

• neutral cisplatin can also diffuse directly across the membrane

3. inside the cell: chloride drops → cisplatin activates

• intracellular [Cl] is much lower (3-20mM)

• in the low Cl environment, cisplatin undergoes hydrolysis (chloride ligands replaced by water)

• activated cisplatin becomes positively charged and highly reactive (this activated form can now bind to biological molecules)

4. cellular fates of activated cisplatin

a. binding to DNA (therapeutic effect)

• activated cisplatin travels into the nucleus

• it forms DNA adducts → causes DNA damage → triggers apoptosis

alternative fate of cisplatin (resistance mechanism)

binding to detoxifying molecules

i. glutathione (GSH) 

• glutathione (Glu-Cys-Gly) can bind activated cisplatin 

• GST enzymes (glutathione-S-transferases) catalyze this

• this produces cisplatin-GSH conjugates, which detoxify the drug

ii. Metallothionein

• metallothionein bind metals and can sequester cisplatin, preventing it from reaching DNA

Efflux: pumping cisplatin out (contributes to resistance)

cisplatin-GSH conjugates are exported by:

a. ATP7A/ATP7B (copper transporters_

• they transport cisplatin out of the cell or into vesicles

b. GS-X/MRP2 pump (multidrug resistance protein)

• actively pumps cisplatin-conjugates outside the cell → excretion

this efflux decreases intracellular cisplatin → contributes to drug resistance

non-covalent DNA binding agents

antibiotics extracted from the soil microbe streptomyces that have anti-tumour activity form tight drug-DNA interactions

mechanism of action of non-covalent DNA binding agents

• DNA intercalation

• free radical DNA damage

• DNA unwinding, impaired synthesis, single and double strand breaks

Bleomycin

forms Bleomycin-Fe(II) complex that interacts with oxygen

oxidation of complex → free radicals (O₂^-, OH) → DNA strand breakage & damage of other cellular constituents

how is Bleomycin administered

through variety of routes (IV, IM, SC), typically in combination with other drugs, to treat lymphomas and cervical, ovarian and testicular cancers

adverse effects of Bleomycins

• pulmonary fibrosis

• anaphylaxis

• GI disturbances

• alopecia

Resistance mechanisms of Bleomycin

• Increased DNA repair

• Increased drug efflux

• Increased expression fo antioxidants or bleomycin hydrolase

Cyclophosphamide, methotrexate & florouracil (CMF)

commonly used regimen of breast cancer chemotherapy that combines three anti-cancer agents

regimen for CMF treatment

four-week cycle

on days 1 and 8 methotrexate and fluorouracil are given IV. Cyclophosphamide sometimes administered IV in conjunction with these drugs, or is taken as an oral tablet once a day for the first 14 days of each cycle

typical treatment involves 6-8 cycles

Antimetabolites

affect cell proliferation by interfering with DNA and RNA synthesis, thus, most effective in S-phase of cell cycle

interfere with the availability of purines and pyrimidines

major classes of antimetabolites

folate antagonists (methotrexate) 

Pyrimidine analogs (5-fluorouracil)

purine analogs 

sugar-modified analogs

how does methotrexate enter cell

via active transport

how does methotrexate work

folic acid inhibitor: structurally similar to folate & binds to dihydrofolate (DHF) reductase

reduces purine and pyrimidine synthesis, thus affects RNA, DNA, and protein synthesis

Methotrexate-polyglutamate metabolites retained in cells to further inhibit RNA/DNA synthesis

what is methotrexate used with

usually used in combination with other drugs for a variety of carcinomas, leukemias and lymphomas

how is methotrexate administered

orally or via injection (IV, IM, SC, IT)

resistance mechanisms of methotrexate

• decreased cellular uptake

• increased DHF reductase expression 

• decreased binding to DHF reductase

• increased efflux

adverse effects of methotrexate

• bone marrow suppression

• GI distress, alopecia

• liver damage (long term treatment) 

• renal damage (high dose)

drug interactions with methotrexate

• aminoglycosides decreases methotrexate absorption

• NSAIDs, penicillins, cephalosporins, cisplatin, probenecid decrease methotrexate absorption

5-fluorouracil

carrier mediated transport into the cell

converted to ribosyl and deoxyrybosyl nucleotide metabolites and incorporated into RNA and DNA

inhibits thymidylate synthase (TS) → decreases thymidine synthesis → decreases DNA synthesis

what is 5-fluorouracil used for

primarily used to treat carcinomas of breast, skin, and GI tract (esophageal, gastric, colorectal, anal)

how is 5-fluorouracil administered

IV or topically

adverse effects of 5-fluorouracil

• bone marrow suppression

• GI disturbances (nausea, vomiting, diarrhea, ulceration)

• alopecia

• cardiotoxicity (angina, arrhythmias) 

• skin irritation

dosing concerns of 5-fluorouracil

• minimum effective dose and maximum tolerated dose very close

• under and overdosing are both concerns 

• monitoring serum levels being investigated to maximize efficacy and minimize adverse effects

resistance mechanisms of 5-fluorouracil

• decreased cell uptake

• increased 5-fluorouracil metabolism

• decreased conversion to nucleotide metabolite 

• increased thymidylate synthase activity

• prolonged DNA synthesis time (DNA repair)